Determination of Word Line to Local Source Line Shorts

ABSTRACT

A number of techniques for determining defects in non-volatile memory arrays are presented, which are particularly applicable to 3D NAND memory, such as that of the BiCS type. Word line to word shorts within a memory block are determined by application of an AC stress mode, followed by a defect detection operation. An inter-block stress and detection operation can be used determine word line to word line leaks between different blocks. Select gate leak line leakage, both the word lines and other select lines, is consider, as are shorts from word lines and select lines to local source lines. In addition to word line and select line defects, techniques for determining shorts between bit lines and low voltage circuitry, as in the sense amplifiers, are presented.

BACKGROUND

This application relates to the operation of re-programmablenon-volatile memory systems such as semiconductor flash memory thatrecord data using charge stored in charge storage elements of memorycells.

Solid-state memory capable of nonvolatile storage of charge,particularly in the form of EEPROM and flash EEPROM packaged as a smallform factor card, has recently become the storage of choice in a varietyof mobile and handheld devices, notably information appliances andconsumer electronics products. Unlike RAM (random access memory) that isalso solid-state memory, flash memory is non-volatile, and retains itsstored data even after power is turned off. Also, unlike ROM (read onlymemory), flash memory is rewritable similar to a disk storage device. Inspite of the higher cost, flash memory is increasingly being used inmass storage applications.

Flash EEPROM is similar to EEPROM (electrically erasable andprogrammable read-only memory) in that it is a non-volatile memory thatcan be erased and have new data written or “programmed” into theirmemory cells. Both utilize a floating (unconnected) conductive gate, ina field effect transistor structure, positioned over a channel region ina semiconductor substrate, between source and drain regions. A controlgate is then provided over the floating gate. The threshold voltagecharacteristic of the transistor is controlled by the amount of chargethat is retained on the floating gate. That is, for a given level ofcharge on the floating gate, there is a corresponding voltage(threshold) that must be applied to the control gate before thetransistor is turned “on” to permit conduction between its source anddrain regions. Flash memory such as Flash EEPROM allows entire blocks ofmemory cells to be erased at the same time.

The floating gate can hold a range of charges and therefore can beprogrammed to any threshold voltage level within a threshold voltagewindow. The size of the threshold voltage window is delimited by theminimum and maximum threshold levels of the device, which in turncorrespond to the range of the charges that can be programmed onto thefloating gate. The threshold window generally depends on the memorydevice's characteristics, operating conditions and history. Eachdistinct, resolvable threshold voltage level range within the windowmay, in principle, be used to designate a definite memory state of thecell.

In order to improve read and program performance, multiple chargestorage elements or memory transistors in an array are read orprogrammed in parallel. Thus, a “page” of memory elements are read orprogrammed together. In existing memory architectures, a row typicallycontains several interleaved pages or it may constitute one page. Allmemory elements of a page are read or programmed together.

Nonvolatile memory devices are also manufactured from memory cells witha dielectric layer for storing charge. Instead of the conductivefloating gate elements described earlier, a dielectric layer is used.Such memory devices utilizing dielectric storage element have beendescribed by Eitan et al., “NROM: A Novel Localized Trapping, 2-BitNonvolatile Memory Cell,” IEEE Electron Device Letters, vol. 21, no. 11,November 2000, pp. 543-545. An ONO dielectric layer extends across thechannel between source and drain diffusions. The charge for one data bitis localized in the dielectric layer adjacent to the drain, and thecharge for the other data bit is localized in the dielectric layeradjacent to the source. For example, U.S. Pat. Nos. 5,768,192 and6,011,725 disclose a nonvolatile memory cell having a trappingdielectric sandwiched between two silicon dioxide layers. Multi-statedata storage is implemented by separately reading the binary states ofthe spatially separated charge storage regions within the dielectric.

SUMMARY OF THE INVENTION

A first set of aspects concern a method of operating a memory circuithaving an array of multiple blocks of non-volatile memory cells formedalong word-lines, a method of determining whether one or more of theword-lines are defective. The method includes performing an intra-blockstress operation on one or more blocks of the array and subsequentlyperforming a defect determination operation. The stress operation has aplurality of stress cycles where each stress cycle for a selected blockincludes: applying a high voltage level to a first set of one or moreword lines of the selected block while concurrently setting a second setof one or more word lines of the selected block at a low voltage level,where at least one word line of the first set is adjacent to at leastone word line of the second set; and subsequently applying the highvoltage level to the second set of word lines while concurrently settingthe first set of word lines at the low voltage level. The defectdetermination operation includes performing a write operation on theselected block and determining whether the write operation wassuccessful.

Other aspects relate to a method of operating a memory circuit having anarray of multiple blocks of non-volatile memory cells of an array formedalong word-lines, a method of determining whether one or more of theword-lines are defective. The method includes performing an inter-blockstress operation on a pair of physically adjacent blocks of the array,including applying a set of stress voltage levels to word lines of thefirst and second of the pair of blocks to thereby introduce a voltagedifferential between them and subsequently performing a defectdetermination operation. The defect determination operation includes:performing an erase operation on the pair of physically adjacent blocks;determining whether the erase operation was successfully completed;subsequently performing a write operation on the pair of physicallyadjacent blocks; and determining whether the write operation wassuccessful.

Further aspect concern a method of operating a non-volatile memorycircuit including an array of non-volatile memory cells formed accordingto a NAND type architecture and having first and second physicallyadjacent, independently biasable select gate lines. The method includesperforming select gate to select gate stress operation on the first andsecond select lines and subsequently performing a defect determinationoperation on the first and second select lines to determine leakagebetween them. The select gate to select gate stress operation applies aset of stress voltage levels to the first and second select lines tothereby introduce a voltage differential between the first and secondselect lines.

Additional aspects present a method of operating an non-volatile memorycircuit including an array of non-volatile memory cells formed accordingto a NAND type architecture and having a first word line physicallyadjacent to a first select gate line. The method performs word line toselect gate stress operation on the first word line and the first selectgate line, including applying a set of stress voltage levels to thefirst word line and first select gate line to thereby introduce avoltage differential between them. A defect determination operation issubsequently performed on the first word line and first select gate lineto determine leakage between them.

Other aspects relate to a method of determining whether one or moreblocks of a semi-conductor memory device are defective, where the memorydevice is formed of multiple blocks, each formed of a plurality of NANDstrings having multiple memory cells connected in series between one ormore source select gates and one or more drain select gates, the memorycells being connected along word lines and the source and drain selectgates being respectively connected along source and drain select lines,where each of a block's NAND strings are connected though thecorresponding source select gates to a local source line. A stressoperation is performed on a selected block, where the stress operationincludes applying a high voltage to the local source line; andconcurrently setting the word lines, source select lines and drainselect lines to a low voltage. A defect determination operation issubsequently performed, where this includes performing a write operationon the selected blocks and determining whether the write operation wassuccessful.

Other aspects also relate to a method of determining defective bit linesin a non-volatile memory circuit having a plurality of memory cells eachconnectable by an associated one of a plurality of bit lines to one of aplurality of sense amps, each of the bit lines connectable to anassociated sense amp by a corresponding switch. The method includesperforming a bit line stress operation and subsequently performing adefect determination operation. The bit line stress operation includesapplying a high voltage level to the first set of one or more bit lineswhile setting the sense amps associated a second set of one or more bitlines set to a sense amp operating level while the correspondingswitches for the first and second sets are off, where at least one bitline of the first set of bit lines is adjacent to at least one of thesense amps associated with the second set of bit lines. The defectdetermination operation includes: with the corresponding switches closedand the sense amps associated with of the second set of bit lines set tothe sense amp operating level, supplying the first set of bit lines froma high voltage supply; and determining whether the high voltage supplycan maintain the first set of bit lines at a first voltage level.

Various aspects, advantages, features and embodiments of the presentinvention are included in the following description of exemplaryexamples thereof, which description should be taken in conjunction withthe accompanying drawings. All patents, patent applications, articles,other publications, documents and things referenced herein are herebyincorporated herein by this reference in their entirety for allpurposes. To the extent of any inconsistency or conflict in thedefinition or use of terms between any of the incorporated publications,documents or things and the present application, those of the presentapplication shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically the main hardware components of amemory system suitable for implementing the present invention.

FIG. 2 illustrates schematically a non-volatile memory cell.

FIG. 3 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time atfixed drain voltage.

FIG. 4 illustrates schematically a string of memory cells organized intoa NAND string.

FIG. 5 illustrates an example of a NAND array 210 of memory cells,constituted from NAND strings 50 such as that shown in FIG. 4.

FIG. 6 illustrates a page of memory cells, organized in the NANDconfiguration, being sensed or programmed in parallel.

FIGS. 7A-7C illustrate an example of programming a population of memorycells.

FIG. 8 shows an example of a physical structure of a 3-D NAND string.

FIGS. 9-12 look at a particular monolithic three dimensional (3D) memoryarray of the NAND type (more specifically of the “BiCS” type).

FIG. 13 is a side view of a block, similar to FIG. 11, but with some ofthe features highlighted.

FIG. 14 shows the toggling of voltage levels being applied to the twosets of word lines to apply an AC stress.

FIGS. 15 and 16 show an exemplary flow for an on-chip AC stress anddefect determination process for word line to word line shorts within ablock.

FIG. 17 is a schematic representation of some of the elements on thememory chip that are involved in the process of FIGS. 15 and 16.

FIG. 18 is a top level, top down diagram of how blocks are paired andplaced next to each other in the exemplary embodiment of FIGS. 9-12.

FIGS. 19 and 20 schematically represents two example of the word linelevels as applied to even and odd block in an inter-block stress mode,

FIGS. 21A and 21B are a schematic representation of some of the elementson the memory chip involved in an inter-block word line shortdetermination process.

FIG. 22 is an oblique view of a simplified version of one block withfour fingers, where each NAND string only has two memory cells and onlya single select gate at either end.

FIGS. 23 and 24 illustrate the consequences of a short between two wordlines of the same block and a short between select gates.

FIG. 25 shows a side view of the exemplary embodiment with an overviewof the applied voltages to stress for local interconnect to word linephysical shorts.

FIGS. 26 and 27 are an exemplary flow of a test mode to screen localinterconnect to word line physical shorts.

FIG. 28 is a schematic representation of some of the elements on thememory chip that are involved in the process of FIGS. 26 and 27.

FIG. 29 is a schematic representation of a possible bit line to lowvoltage short.

FIG. 30 is similar to FIG. 29, but marked with some of the voltagelevels involved in the stress phase of determining bit line shorts.

FIG. 31 is an exemplary flow for test process for determining bit lineto low voltage shorts.

FIG. 32 is a schematic representation of some of the elements on thememory chip involved in the process of FIG. 31B.

DETAILED DESCRIPTION Memory System

FIG. 1 illustrates schematically the main hardware components of amemory system suitable for implementing the present invention. Thememory system 90 typically operates with a host 80 through a hostinterface. The memory system may be in the form of a removable memorysuch as a memory card, or may be in the form of an embedded memorysystem. The memory system 90 includes a memory 102 whose operations arecontrolled by a controller 100. The memory 102 comprises one or morearray of non-volatile memory cells distributed over one or moreintegrated circuit chip. The controller 100 may include interfacecircuits 110, a processor 120, ROM (read-only-memory) 122, RAM (randomaccess memory) 130, programmable nonvolatile memory 124, and additionalcomponents. The controller is typically formed as an ASIC (applicationspecific integrated circuit) and the components included in such an ASICgenerally depend on the particular application.

With respect to the memory section 102, semiconductor memory devicesinclude volatile memory devices, such as dynamic random access memory(“DRAM”) or static random access memory (“SRAM”) devices, non-volatilememory devices, such as resistive random access memory (“ReRAM”),electrically erasable programmable read only memory (“EEPROM”), flashmemory (which can also be considered a subset of EEPROM), ferroelectricrandom access memory (“FRAM”), and magnetoresistive random access memory(“MRAM”), and other semiconductor elements capable of storinginformation. Each type of memory device may have differentconfigurations. For example, flash memory devices may be configured in aNAND or a NOR configuration.

The memory devices can be formed from passive and/or active elements, inany combinations. By way of non-limiting example, passive semiconductormemory elements include ReRAM device elements, which in some embodimentsinclude a resistivity switching storage element, such as an anti-fuse,phase change material, etc., and optionally a steering element, such asa diode, etc. Further by way of non-limiting example, activesemiconductor memory elements include EEPROM and flash memory deviceelements, which in some embodiments include elements containing a chargestorage region, such as a floating gate, conductive nanoparticles, or acharge storage dielectric material.

Multiple memory elements may be configured so that they are connected inseries or so that each element is individually accessible. By way ofnon-limiting example, flash memory devices in a NAND configuration (NANDmemory) typically contain memory elements connected in series. A NANDmemory array may be configured so that the array is composed of multiplestrings of memory in which a string is composed of multiple memoryelements sharing a single bit line and accessed as a group.Alternatively, memory elements may be configured so that each element isindividually accessible, e.g., a NOR memory array. NAND and NOR memoryconfigurations are exemplary, and memory elements may be otherwiseconfigured.

The semiconductor memory elements located within and/or over a substratemay be arranged in two or three dimensions, such as a two dimensionalmemory structure or a three dimensional memory structure.

In a two dimensional memory structure, the semiconductor memory elementsare arranged in a single plane or a single memory device level.Typically, in a two dimensional memory structure, memory elements arearranged in a plane (e.g., in an x-z direction plane) which extendssubstantially parallel to a major surface of a substrate that supportsthe memory elements. The substrate may be a wafer over or in which thelayer of the memory elements are formed or it may be a carrier substratewhich is attached to the memory elements after they are formed. As anon-limiting example, the substrate may include a semiconductor such assilicon.

The memory elements may be arranged in the single memory device level inan ordered array, such as in a plurality of rows and/or columns.However, the memory elements may be arrayed in non-regular ornon-orthogonal configurations. The memory elements may each have two ormore electrodes or contact lines, such as bit lines and word lines.

A three dimensional memory array is arranged so that memory elementsoccupy multiple planes or multiple memory device levels, thereby forminga structure in three dimensions (i.e., in the x, y and z directions,where the y direction is substantially perpendicular and the x and zdirections are substantially parallel to the major surface of thesubstrate).

As a non-limiting example, a three dimensional memory structure may bevertically arranged as a stack of multiple two dimensional memory devicelevels. As another non-limiting example, a three dimensional memoryarray may be arranged as multiple vertical columns (e.g., columnsextending substantially perpendicular to the major surface of thesubstrate, i.e., in the y direction) with each column having multiplememory elements in each column. The columns may be arranged in a twodimensional configuration, e.g., in an x-z plane, resulting in a threedimensional arrangement of memory elements with elements on multiplevertically stacked memory planes. Other configurations of memoryelements in three dimensions can also constitute a three dimensionalmemory array.

By way of non-limiting example, in a three dimensional NAND memoryarray, the memory elements may be coupled together to form a NAND stringwithin a single horizontal (e.g., x-z) memory device levels.Alternatively, the memory elements may be coupled together to form avertical NAND string that traverses across multiple horizontal memorydevice levels. Other three dimensional configurations can be envisionedwherein some NAND strings contain memory elements in a single memorylevel while other strings contain memory elements which span throughmultiple memory levels. Three dimensional memory arrays may also bedesigned in a NOR configuration and in a ReRAM configuration.

Typically, in a monolithic three dimensional memory array, one or morememory device levels are formed above a single substrate. Optionally,the monolithic three dimensional memory array may also have one or morememory layers at least partially within the single substrate. As anon-limiting example, the substrate may include a semiconductor such assilicon. In a monolithic three dimensional array, the layersconstituting each memory device level of the array are typically formedon the layers of the underlying memory device levels of the array.However, layers of adjacent memory device levels of a monolithic threedimensional memory array may be shared or have intervening layersbetween memory device levels.

Then again, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device having multiplelayers of memory. For example, non-monolithic stacked memories can beconstructed by forming memory levels on separate substrates and thenstacking the memory levels atop each other. The substrates may bethinned or removed from the memory device levels before stacking, but asthe memory device levels are initially formed over separate substrates,the resulting memory arrays are not monolithic three dimensional memoryarrays. Further, multiple two dimensional memory arrays or threedimensional memory arrays (monolithic or non-monolithic) may be formedon separate chips and then packaged together to form a stacked-chipmemory device.

Associated circuitry is typically required for operation of the memoryelements and for communication with the memory elements. As non-limitingexamples, memory devices may have circuitry used for controlling anddriving memory elements to accomplish functions such as programming andreading. This associated circuitry may be on the same substrate as thememory elements and/or on a separate substrate. For example, acontroller for memory read-write operations may be located on a separatecontroller chip and/or on the same substrate as the memory elements.

One of skill in the art will recognize that this invention is notlimited to the two dimensional and three dimensional exemplarystructures described but cover all relevant memory structures within thespirit and scope of the invention as described herein and as understoodby one of skill in the art.

Physical Memory Structure

FIG. 2 illustrates schematically a non-volatile memory cell. The memorycell 10 can be implemented by a field-effect transistor having a chargestorage unit 20, such as a floating gate or a charge trapping(dielectric) layer. The memory cell 10 also includes a source 14, adrain 16, and a control gate 30.

There are many commercially successful non-volatile solid-state memorydevices being used today. These memory devices may employ differenttypes of memory cells, each type having one or more charge storageelement.

Typical non-volatile memory cells include EEPROM and flash EEPROM.Examples of EEPROM cells and methods of manufacturing them are given inU.S. Pat. No. 5,595,924. Examples of flash EEPROM cells, their uses inmemory systems and methods of manufacturing them are given in U.S. Pat.Nos. 5,070,032, 5,095,344, 5,315,541, 5,343,063, 5,661,053, 5,313,421and 6,222,762. In particular, examples of memory devices with NAND cellstructures are described in U.S. Pat. Nos. 5,570,315, 5,903,495,6,046,935. Also, examples of memory devices utilizing dielectric storageelements have been described by Eitan et al., “NROM: A Novel LocalizedTrapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Letters,vol. 21, no. 11, November 2000, pp. 543-545, and in U.S. Pat. Nos.5,768,192 and 6,011,725.

In practice, the memory state of a cell is usually read by sensing theconduction current across the source and drain electrodes of the cellwhen a reference voltage is applied to the control gate. Thus, for eachgiven charge on the floating gate of a cell, a corresponding conductioncurrent with respect to a fixed reference control gate voltage may bedetected. Similarly, the range of charge programmable onto the floatinggate defines a corresponding threshold voltage window or a correspondingconduction current window.

Alternatively, instead of detecting the conduction current among apartitioned current window, it is possible to set the threshold voltagefor a given memory state under test at the control gate and detect ifthe conduction current is lower or higher than a threshold current(cell-read reference current). In one implementation the detection ofthe conduction current relative to a threshold current is accomplishedby examining the rate the conduction current is discharging through thecapacitance of the bit line.

FIG. 3 illustrates the relation between the source-drain current I_(D)and the control gate voltage V_(CG) for four different charges Q1-Q4that the floating gate may be selectively storing at any one time. Withfixed drain voltage bias, the four solid I_(D) versus V_(CG) curvesrepresent four of seven possible charge levels that can be programmed ona floating gate of a memory cell, respectively corresponding to fourpossible memory states. As an example, the threshold voltage window of apopulation of cells may range from 0.5V to 3.5V. Seven possibleprogrammed memory states “0”, “1”, “2”, “3”, “4”, “5”, “6”, and anerased state (not shown) may be demarcated by partitioning the thresholdwindow into regions in intervals of 0.5V each. For example, if areference current, IREF of 2 μA is used as shown, then the cellprogrammed with Q1 may be considered to be in a memory state “1” sinceits curve intersects with I_(REF) in the region of the threshold windowdemarcated by VCG=0.5V and 1.0V. Similarly, Q4 is in a memory state “5”.

As can be seen from the description above, the more states a memory cellis made to store, the more finely divided is its threshold window. Forexample, a memory device may have memory cells having a threshold windowthat ranges from −1.5V to 5V. This provides a maximum width of 6.5V. Ifthe memory cell is to store 16 states, each state may occupy from 200 mVto 300 mV in the threshold window. This will require higher precision inprogramming and reading operations in order to be able to achieve therequired resolution.

NAND Structure

FIG. 4 illustrates schematically a string of memory cells organized intoa NAND string. A NAND string 50 comprises a series of memory transistorsM1, M2, . . . Mn (e.g., n=4, 8, 16 or higher) daisy-chained by theirsources and drains. A pair of select transistors S1, S2 controls thememory transistor chain's connection to the external world via the NANDstring's source terminal 54 and drain terminal 56 respectively. In amemory array, when the source select transistor S1 is turned on, thesource terminal is coupled to a source line (see FIG. 5). Similarly,when the drain select transistor S2 is turned on, the drain terminal ofthe NAND string is coupled to a bit line of the memory array. Eachmemory transistor 10 in the chain acts as a memory cell. It has a chargestorage element 20 to store a given amount of charge so as to representan intended memory state. A control gate 30 of each memory transistorallows control over read and write operations. As will be seen in FIG.5, the control gates 30 of corresponding memory transistors of a row ofNAND string are all connected to the same word line. Similarly, acontrol gate 32 of each of the select transistors S1, S2 providescontrol access to the NAND string via its source terminal 54 and drainterminal 56 respectively. Likewise, the control gates 32 ofcorresponding select transistors of a row of NAND string are allconnected to the same select line.

When an addressed memory transistor 10 within a NAND string is read oris verified during programming, its control gate 30 is supplied with anappropriate voltage. At the same time, the rest of the non-addressedmemory transistors in the NAND string 50 are fully turned on byapplication of sufficient voltage on their control gates. In this way, aconductive path is effectively created from the source of the individualmemory transistor to the source terminal 54 of the NAND string andlikewise for the drain of the individual memory transistor to the drainterminal 56 of the cell. Memory devices with such NAND string structuresare described in U.S. Pat. Nos. 5,570,315, 5,903,495, 6,046,935.

FIG. 4B illustrates an example of a NAND array 210 of memory cells,constituted from NAND strings 50 such as that shown in FIG. 4. Alongeach column of NAND strings, a bit line such as bit line 36 is coupledto the drain terminal 56 of each NAND string. Along each bank of NANDstrings, a source line such as source line 34 is coupled to the sourceterminals 54 of each NAND string. Also the control gates along a row ofmemory cells in a bank of NAND strings are connected to a word line suchas word line 42. The control gates along a row of select transistors ina bank of NAND strings are connected to a select line such as selectline 44. An entire row of memory cells in a bank of NAND strings can beaddressed by appropriate voltages on the word lines and select lines ofthe bank of NAND strings.

FIG. 6 illustrates a page of memory cells, organized in the NANDconfiguration, being sensed or programmed in parallel. FIG. 6essentially shows a bank of NAND strings 50 in the memory array 210 ofFIG. 5, where the detail of each NAND string is shown explicitly as inFIG. 4. A physical page, such as the page 60, is a group of memory cellsenabled to be sensed or programmed in parallel. This is accomplished bya corresponding page of sense amplifiers 212. The sensed results arelatched in a corresponding set of latches 214. Each sense amplifier canbe coupled to a NAND string via a bit line. The page is enabled by thecontrol gates of the cells of the page connected in common to a wordline 42 and each cell accessible by a sense amplifier accessible via abit line 36. As an example, when respectively sensing or programming thepage of cells 60, a sensing voltage or a programming voltage isrespectively applied to the common word line WL3 together withappropriate voltages on the bit lines.

Physical Organization of the Memory

One important difference between flash memory and other of types ofmemory is that a cell must be programmed from the erased state. That isthe floating gate must first be emptied of charge. Programming then addsa desired amount of charge back to the floating gate. It does notsupport removing a portion of the charge from the floating gate to gofrom a more programmed state to a lesser one. This means that updateddata cannot overwrite existing data and must be written to a previousunwritten location.

Furthermore erasing is to empty all the charges from the floating gateand generally takes appreciable time. For that reason, it will becumbersome and very slow to erase cell by cell or even page by page. Inpractice, the array of memory cells is divided into a large number ofblocks of memory cells. As is common for flash EEPROM systems, the blockis the unit of erase. That is, each block contains the minimum number ofmemory cells that are erased together. While aggregating a large numberof cells in a block to be erased in parallel will improve eraseperformance, a large size block also entails dealing with a largernumber of update and obsolete data.

Each block is typically divided into a number of physical pages. Alogical page is a unit of programming or reading that contains a numberof bits equal to the number of cells in a physical page. In a memorythat stores one bit per cell, one physical page stores one logical pageof data. In memories that store two bits per cell, a physical pagestores two logical pages. The number of logical pages stored in aphysical page thus reflects the number of bits stored per cell. In oneembodiment, the individual pages may be divided into segments and thesegments may contain the fewest number of cells that are written at onetime as a basic programming operation. One or more logical pages of dataare typically stored in one row of memory cells. A page can store one ormore sectors. A sector includes user data and overhead data.

All-Bit, Full-Sequence MLC Programming

FIG. 7A-7C illustrate an example of programming a population of 4-statememory cells. FIG. 7A illustrates the population of memory cellsprogrammable into four distinct distributions of threshold voltagesrespectively representing memory states “0”, “1”, “2” and “3”. FIG. 7Billustrates the initial distribution of “erased” threshold voltages foran erased memory. FIG. 6C illustrates an example of the memory aftermany of the memory cells have been programmed. Essentially, a cellinitially has an “erased” threshold voltage and programming will move itto a higher value into one of the three zones demarcated by verifylevels vV₁, vV₂ and vV₃. In this way, each memory cell can be programmedto one of the three programmed states “1”, “2” and “3” or remainun-programmed in the “erased” state. As the memory gets moreprogramming, the initial distribution of the “erased” state as shown inFIG. 7B will become narrower and the erased state is represented by the“0” state.

A 2-bit code having a lower bit and an upper bit can be used torepresent each of the four memory states. For example, the “0”, “1”, “2”and “3” states are respectively represented by “11”, “01”, “00” and‘10”. The 2-bit data may be read from the memory by sensing in“full-sequence” mode where the two bits are sensed together by sensingrelative to the read demarcation threshold values rV₁, rV₂ and rV₃ inthree sub-passes respectively.

3-D NAND Structures

An alternative arrangement to a conventional two-dimensional (2-D) NANDarray is a three-dimensional (3-D) array. In contrast to 2-D NANDarrays, which are formed along a planar surface of a semiconductorwafer, 3-D arrays extend up from the wafer surface and generally includestacks, or columns, of memory cells extending upwards. Various 3-Darrangements are possible. In one arrangement a NAND string is formedvertically with one end (e.g. source) at the wafer surface and the otherend (e.g. drain) on top. In another arrangement a NAND string is formedin a U-shape so that both ends of the NAND string are accessible on top,thus facilitating connections between such strings. Examples of suchNAND strings and their formation are described in U.S. PatentPublication Number 2012/0220088 and in U.S. Patent Publication Number2013/0107628, which are hereby incorporated by reference in theirentirety.

FIG. 8 shows a first example of a NAND string 701 that extends in avertical direction, i.e. extending in the z-direction, perpendicular tothe x-y plane of the substrate. Memory cells are formed where a verticalbit line (local bit line) 703 passes through a word line (e.g. WL0, WL1,etc.). A charge trapping layer between the local bit line and the wordline stores charge, which affects the threshold voltage of thetransistor formed by the word line (gate) coupled to the vertical bitline (channel) that it encircles. Such memory cells may be formed byforming stacks of word lines and then etching memory holes where memorycells are to be formed. Memory holes are then lined with a chargetrapping layer and filled with a suitable local bit line/channelmaterial (with suitable dielectric layers for isolation).

As with planar NAND strings, select gates 705, 707, are located ateither end of the string to allow the NAND string to be selectivelyconnected to, or isolated from, external elements 709, 711. Suchexternal elements are generally conductive lines such as common sourcelines or bit lines that serve large numbers of NAND strings. VerticalNAND strings may be operated in a similar manner to planar NAND stringsand both SLC and MLC operation is possible. While FIG. 8 shows anexample of a NAND string that has 32 cells (0-31) connected in series,the number of cells in a NAND string may be any suitable number. Not allcells are shown for clarity. It will be understood that additional cellsare formed where word lines 3-29 (not shown) intersect the localvertical bit line.

A 3D NAND array can, loosely speaking, be formed tilting up therespective structures 50 and 210 of FIGS. 5 and 6 to be perpendicular tothe x-y plane. In this example, each y-z plane corresponds to the pagestructure of FIG. 6, with m such plane at differing x locations. The(global) bit lines, BL1-m, each run across the top to an associatedsense amp SA1-m. The word lines, WL1-n, and source and select linesSSL1-n and DSL1-n, then run in x direction, with the NAND stringconnected at bottom to a common source line CSL.

FIGS. 9-12 look at a particular monolithic three dimensional (3D) memoryarray of the NAND type (more specifically of the “BiCS” type), where oneor more memory device levels are formed above a single substrate, inmore detail. FIG. 9 is an oblique projection of part of such astructure, showing a portion corresponding to two of the page structuresin FIG. 5, where, depending on the embodiment, each of these couldcorrespond to a separate block or be different “fingers” of the sameblock. Here, instead to the NAND strings lying in a common y-z plane,they are squashed together in the y direction, so that the NAND stringsare somewhat staggered in the x direction. On the top, the NAND stringsare connected along global bit lines (BL) spanning multiple suchsub-divisions of the array that run in the x direction. Here, globalcommon source lines (SL) also run across multiple such structures in thex direction and are connect to the sources at the bottoms of the NANDstring, which are connected by a local interconnect (LI) that serves asthe local common source line of the individual finger. Depending on theembodiment, the global source lines can span the whole, or just aportion, of the array structure. Rather than use the local interconnect(LI), variations can include the NAND string being formed in a U typestructure, where part of the string itself runs back up, such as isdescribed in U.S. patent application Ser. No. 13/927,659, filed on Jun.26, 2013.

To the right of FIG. 9 is a representation of the elements of one of thevertical NAND strings from the structure to the left. Multiple memorycells are connected through a drain select gate SGD to the associatedbit line BL at the top and connected through the associated sourceselect gate SDS to the associated local source line LI to a globalsource line SL. It is often useful to have a select gate with a greaterlength than that of memory cells, where this can alternately be achievedby having several select gates in series (as described in U.S. patentapplication Ser. No. 13/925,662, filed on Jun. 24, 2013), making formore uniform processing of layers. Additionally, the select gates areprogrammable to have their threshold levels adjusted, aspects of whichare described in US patent publication number 2014-0169095. Thisexemplary embodiment also includes several dummy cells at the ends thatare not used to store user data, as their proximity to the select gatesmakes them more prone to disturbs.

FIG. 10 shows a top view of the structure for two blocks in theexemplary embodiment. Two blocks (BLK0 above, BLK1 below) are shown,each having four fingers that run left to right. The word lines andselect gate lines of each level also run left to right, with the wordlines of the different fingers of the same block being commonlyconnected at a “terrace” and then on to receive their various voltagelevel through the word line select gates at WLTr. The word lines of agiven layer in a block can also be commonly connected on the far sidefrom the terrace. The selected gate lines can be individual for eachlevel, rather common, allowing the fingers to be individually selected.The bit lines are shown running up and down the page and connect on tothe sense amp circuits, where, depending on the embodiment, each senseamp can correspond to a single bit line or be multiplexed to several bitlines.

FIG. 11 shows a side view of one block, again with four fingers. In thisexemplary embodiment, the select gates SGD and SGS at either end of theNAND strings are formed of four layers, with the word lines WLin-between, all formed over a CPWELL. A given finger is selected bysetting its select gates to a level VSG and the word lines are biasedaccording to the operation, such as a read voltage (VCGRV) for theselected word lines and the read-pass voltage (VREAD) for thenon-selected word lines. The non-selected fingers can then be cut off bysetting their select gates accordingly.

FIG. 12 illustrates some detail of an individual cell. A dielectric coreruns in the vertical direction and is surrounded by a channel siliconlayer, that is in turn surrounded a tunnel dielectric (TNL) and then thecharge trapping dielectric layer (CTL). The gate of the cell is hereformed of tungsten with which is surrounded by a metal barrier and isseparated from the charge trapping layer by blocking (BLK) oxide and ahigh K layer.

Array Structure Defects

Memory arrays such as those described above are often subject to variousdefects, such as broken or leaky word lines and bit lines. A number oftechniques for the determining of, and the dealing with, these sorts ofproblems are presented in the following US patent publications:2012-0008405; 2012-0008384; 2012-0008410; 2012-0281479; 2013-0031429;2013-0031429; and US2013-0229868. The following present a number ofadditional techniques in the context of the sort of 3D memory structuresdescribed above with respect to FIGS. 9-12. Although these techniquesare particularly applicable to such structures, many of them are moregenerally applicable, such as to 2D NAND and other array structures.

Word Line to Word Line Shorts, Same Block: AC Stress Mode

This section considers shorts between word lines between of the sameblock, whether in a 2D array or a 3D. In a 3D arrangement, such asillustrated in the FIGS. 9-12, a number of word lines are stacked on topof each other, with an oxide layer in between the pairs of word lines toact as an insulating layer. If this oxide layer is not depositeduniformly or, due to some contamination, is thinner than the targetvalues, the oxide can fail at some point, leading to a short. The stressmode described in this section can accelerate the failure to occur aspart of a defect determination process. FIG. 13 can help to illustratethe problem.

FIG. 13 is a side view of a block, similar to FIG. 11, but with some ofthe features relevant to the current discussion highlighted. As before,four fingers of a block are shown with some of the global bit lines,source lines and so on running across the top. More specifically, thetwo arrows to the right show two of the gaps between word lines thatwould be filled with oxide. (As discussed further below, these oxidelayers are also between select gate lines, as shown in the secondfinger.) The potential word line to word line shorts would then beacross these oxide layers, such as illustrated at the right most finger.

A number of references cited above present techniques for determiningsuch shorts. Typically, these use a DC stress applied to the word lines.This section uses an AC stress that, in the exemplary embodiment, isapplied to the odd and even word lines; for example, while toggling theeven word lines to a high voltage level VH, the odd word lines aretoggled to a low voltage level VL. (More generally, this can be done towith two sets of word lines, such as a portion of the word lines, wherethe two sets have at least one word line from each that is adjacent.) VHis taken as a high voltage level such that the Delta (VH-VL) is highenough to stress the oxide in between adjacent WLs. For example, VH canbe as high as 20V to reflect the sort of word line stress levels seendue to program and erase voltage levels used on the device. VL can bethe low level (ground or VSS) on the device. This toggling isillustrated schematically in FIG. 14, that shows the voltage levelsbeing applied to the two sets of word lines.

As shown in FIG. 14, the even word lines alternately have the VL and VHlevels applied, with the odd word lines being similarly toggled but withthe phase reversed. These waveforms can be based on a number of settableparameters, including the time period of the VH level, defined byparameter Th, and the time period of VL level, defined by parameter TL.Th and TL can be adjusted to have Delta (VH-VL) between adjacent WLs fora fixed duration. In case the rise or the fall time of VH and VL differ,the parameters Th and TL can be adjusted to achieve a desired durationof the stress in each cycle. The number of loops of this AC stress canalso be parameterized (loop count parameter) to prevent any kind of overkill or under kill. Although the exemplary embodiment applies theselevels along the sets of even and odd word lines, as noted above thiscan be done on other subsets that have one or more adjacent word lineswithin the block or finger.

FIGS. 15 and 16 are an exemplary flow for the on-chip stress (FIG. 15)and defect determination (FIG. 16) process. The process begins with thetest mode entry at 1501. This will typically be part of an initial testprocess, done before a device is packaged or shipped, to determinedefective die, but also can also be done after the device has been inoperation for some time, where this could them be implemented at thesystem level and with possible adjustments to the parameters (see 1509).At 1503 it is determined whether to just do a single block test ormultiple, even all, blocks. In other embodiments, partial block test canalso be done. The block (1507) or blocks (1505) are then selected. Thestress is then applied at 1509, which show the parameters involved. Theidea is to apply the stress to break any weak spots during the test,basically forcing the issue, rather than wait for these failures tooccur during operation.

After the stress operation comes the defect determination operation. Anumber of techniques for this are described in US patent publications:2012-0008405; 2012-0008384; 2012-0008410; 2012-0281479; 2013-0031429;2013-0031429; and US2013-0229868. For the exemplary flow here, thestressed block or blocks are programmed and then can be read back tocheck for failures. The flow picks up at 1601 with a program operation,the status of which is monitored at 1603. If the write operation fails(1605), the bad block or blocks are marked as such and not used.Alternately, this (as well as 1611 below) could also be done at the wordline level. If the program operation passes, the detection can then moveon to a read operation at 1607. The read status is monitored at 1609 by,for example, comparing the data as read back with the data that wasoriginally programmed. If the read status comes back as a fail, then thebad block or blocks can be marked (1611) as such. If the tested blockspass (1613), the test mode can then exited (1615).

For the defect determination operation, determining whether a programoperation has failed can be based on a word line failing by exceedingmaximum number of program loops or on the read operation coming backwith a failed bit count exceeding a limit. The data written and readback for this process can be a random pattern, either predetermined ornot. In either case, the comparison of the read back data can be basedon a comparison of the data pattern that was to be written, each bymaintaining a copy or because it is based on a known, predeterminedpattern. Rather than a direct comparison, it can alternately (oradditional) be based on ECC and whether the data can be extracted withinthe ECC capabilities.

FIG. 17 is a schematic representation of some of the elements on thememory chip that are involved in this process. A number of differentembodiments are possible, but shows some of the basic elements. Thearray 1701 and its associated decoding and sensing circuitry can be ofthe BiCS or other 3D variety, but is here shown in more of a 2D sort ofrepresentation for simplicity. The memory circuit has a set of 10 pads1703 for commands, addresses and data transfer, which can then be passedon to command and block/page address registers (1705, 1707). Anoscillator 1709 can be used with the clock generator to provide neededclock signals. An finite state machine (FSM) and Sequencer block 1713represented to on-chip control logic that controls the various drivers1715 for the array 1701. References voltage and current generators 1721supply the various reference levels, including those supplied to thecharge pump circuits block 1723, which generates the high level VHapplied to the word lines in the intra-block AC stress mode of thissection. A pulse generation circuit, here schematically separated out asblock 1725, will then supply the stress level to the selected block whenenabled by mode select signal, as schematically represented by thecontrol signals to the control gates of the high voltage switches 1727and 1729.

For any of the variations, these techniques can be effect for detectingcurrent or incipient word line to word line shorts within a given block,without over identification, improving the identified defective partsper million (DPPM) value. This process can be implemented as a built inself-test (BIST) process that can help in reducing test times and alsogives the option to perform the stress at the system level at othertimes, such as before performing a block.

Word Line to Word Line Shorts, Adjacent Blocks

In most non-volatile array structures, the issue of word line to wordline shorts is traditional an issue for word lines within a commonblocks. In some structures, such as the sort of 3D BiCS-type structuredescribed above with respect to FIGS. 9-12, word lines from different,but physically adjacent blocks can be in close proximity. This sectionlooks at the detection of such inter-block word line to word lineshorts. As can be seen from FIG. 9, where the two shown fingers are fromdifferent blocks, word lines on the same vertical level from differentblocks can be closely placed, separated by a distance that will decreaseas device scales continue to shrink.

Word line to word line shorts from adjacent blocks can manifestthemselves as erase disturbs, program disturbs, or both. For example,consider the case where a block X is already in a programmed state andunselected for erase, but an adjacent block, block X+1, is selected forerase. In case of a short between a word line WLn of block X and wordline of block X+1, when applying the erase voltage level to block X+1,the erased bias level will transfer across the shorted word line WLn toblock X, causing some degree of erase there so that block X will losealready programmed data, resulting in erase disturb there. Similarly, ifblock X is already programmed and meant to be unselected for additionalprogramming while the adjacent block X+1 is selected for programming,when a programming pulse is applied along WLn in block X+1, this will betransferred across the short. WLn in block X be get over programmed andWLn of block X+1 will see high loading, resulting in program disturbthere.

FIG. 18 is a top level, top down diagram of how blocks are paired andplaced next to each other in the exemplary embodiment. The four (in thisembodiment) fingers of block 0 1801 are at top, with each word line inthe stack connected to a corresponding terrace level that in addition tobeing to the left of the fingers of block 0, are also in proximity tothe fingers of block 1 1803. To the left and right are the respectivetransfer gates for block 0 (1805) and block 1 (1807). The bit lines thenrun up and down in this view, connecting the NAND strings running intothe page to the associated sense amps. The arrows show some of theexamples of where inter-block word line shorts can occur, such asbetween a word line of one block and the other blocks terrace region1811 or between the word lines of adjacent fingers of different blocks1813. Also this discussion is given in the context of the BiCS typestructure, it can also be applied to other structures where word linesof one block are in proximity to the word lines of another block.

The process for determining inter-block word line to word line shortsbetween adjacent blocks can again use a stress operation and a detectionoperation, where the stress phase can be an AC or DC stress. The highlevel VH is again a high voltage level such that the Delta (VH-VL) ishigh enough to stress the oxide or weak defect in between adjacent wordlines. VH can be as high as 20V to correspond to the stresses the devicewill see during erase and write operations. The low level VL issimilarly such that the Delta (VH-VL) is high enough to stress the oxideor weak defect in between adjacent word lines, where VL can be as low asthe low on-chip level of VSS or ground. After the stress mode, whetherAC or DC, a word line to word line leakage determination can then bedone to check pass/fail status using decoder circuit, modified asappropriate.

The inter-block stress between word lines can be applied in a number ofways. For example, a differing bias can be applied on the word linesfrom even and odd blocks. In one embodiment, this could be applying ahigh voltage to all word lines on even blocks and a low voltage to oddblock, or vice versa. This will create a stress between word lines ofphysically adjacent blocks to be able to weed out defects at time attest time. This stress can be applied to two or more physically adjacentblocks at a time.

FIG. 19 schematically represents the word line levels as applied to evenand odd block under this arrangements, where the word lines on evenblocks are set high and the odd word lines are set low, where this canalso be done the other way around. These levels can then be held forsome duration in a DC stress mode, or toggled over a number of cycles.These levels can then be applied to by the corresponding decodingcircuitry and transfer gates to the word lines according to thispattern.

In an alternate stress mode, a stripe pattern can be applied, either asDC or toggled for a AC mode, to the word lines of adjacent blocks, whereone pattern is applied for one block and the pattern reversed for theadjacent block. For example, on the word lines of even blocks, voltagelevels can be applied in an alternating high-low pattern, as shown inFIG. 20, with the inverted low-high pattern on the odd blocks. Thisstress can again be done a pair of blocks at a time or multiple selectedof blocks.

A post-stress detection sequence can then follow. The exemplaryembodiment for a detection sequence uses an erase disturb test to detectthe word line to word line shorts from adjacent blocks. This can be doneby erasing all blocks of an array. For all blocks that pass the erase, aprogram follows to see whether blocks program correctly. For example,random data can be programmed on all blocks of the array, read back, andcompared against the expected data to check for any program disturb. Ablock N can then be erased, after which an adjacent block N+1 is checkedfor any erase disturb.

FIG. 21A is a schematic representation of some of the elements on thememory chip, similar to FIG. 17 and with the corresponding elementssimilarly numbered (i.e., array 1701 is now array 2101 and so on). Toenable the inter-block word line to word line stress mode, acorresponding stress mode select signal is sent from the on-chip controllogic to the charge pump, which in turn can supply the needed highvoltage level to the control gate drivers in block 2115. These driverscan then supply the high and low stress voltages CGUE and CGUO to theeven and odd blocks. FIG. 21B gives more detail on the decodingcircuitry and its connections to the array for the exemplary embodiment,where the even block terraces are to the left and the odd block terracesto the right.

For any of the variations, the test mode of this section canconsequently be used to catch word line to word line shorts between wordlines of adjacent blocks. By using an AC version of the sort ofalternating high and low voltage pattern illustrated with respect toFIG. 20, this can concurrently apply the sort stress described in thepreceding section for word line to word line shorts within a block,helping to reduce test time. For the voltage patterns of either of FIG.19 or 20, the separate biasing of neighboring block is used, which isnot conventional in NAND memory decoder designs, but which can befurther enhanced for parallelism by using opposite alternating patternson facing blocks. As with the other defect determination methodsdescribed here, the detection of adjacent blocks' erase disturb, programdisturb, or both can help to avoid the corrupting of user data.

Select Gate Shorts

The preceding two sections looking at defects that could to shortsbetween word lines. Going back to the structure shown FIGS. 9 and 10,this has multiple select gates on both the source and drain sides, sothat in each finger there will be oxide layers between the end most wordlines and the adjacent select gates lines (on both the source and drainends) and also between the multiple select gate lines themselves. Therewill also be oxide layers between the corresponding select gate layersin different, adjacent blocks and different adjacent finger. Also, asthe select gates in the exemplary embodiment have tunablethresholds—that is, they are programmable—they will also be subjected tohigh voltage levels. Consequently, select gate to select gate and wordline to select gate shorts can occur.

Although similar to the word line case, the select gate structures havesome differences that can be illustrated with respect to FIG. 22. FIG.22 is an oblique view of a simplified version of one block with fourfingers, where each NAND string only has two memory cells and only asingle select gate at either end. Considering the rightmost finger, thepair of word lines 2221 and 2225 are between the select gate lines 2201and 2211. As discussed above, the word lines of a given level fromdifferent fingers of the same block are connected together, as shown at2223 for word line 2221, along the terrace region. The select lines ofthe fingers, however, are separate, so that the fingers can individuallybe selected. Consequently, the select gate lines at 2201, 2203, 2205,and 2207 are independent controllable, lacking the sort of connectionbetween fingers that 2223 effects for the word lines. The simplifieddrawing of FIG. 22 shows only a single select gate on either end, butwhen there are multiple select gates, these are typically operated inunison as they perform a common select function.

In this structure, the consequences of a select gate to select gateshort can be more severe that a word line to word line short, as can beillustrated by comparing FIGS. 23 and 24. Both of these show four NANDstrings from four different fingers of the same block. A given word lineconnects the gates across the fingers, while the select gates areindividually controllable (where, in these figures, only a single selectgate is shown on each of the source and drain sides). Referring to FIG.23, a short between two word lines (such as WL0 and WL1) would cause allof the pages of data on these word lines of the block to be lost.Referring now to FIG. 24, if instead there is a short between selectgates of different finger (such as SGD0 and SGD1), the correspondingNAND strings cannot be independently accessed, causing the loss of amuch larger number of pages. As with the word line case, there areseveral types of shorts to consider. There can be select gate to selectgate shorts in the same finger of the same block; across differentfingers of the same block; and between blocks. There can also be shortsbetween the end most word lines and adjacent select lines. (A furtherpossible short to the interconnect or local source line, affecting bothword lines and select gate line, is discussed in the next section.) Foreach of these mechanism, a corresponding stress can be applied followedby a detection operation. Either an AC or DC stress can be used and canbe done independently or, when appropriate, can be incorporated into theword line test operations. In some cases, such as the same block,different finger case, that is not available for the word line to wordline case as illustrated in the FIG. 24, this would need to be anindependent stress operation. Further, in addition to the stress anddetection operations described herein, those presented in US patentpublications: 2012-0008405; 2012-0008384; 2012-0008410; 2012-0281479;2013-0031429; 2013-0031429; and US2013-0229868 can also be adapted tothe select gate, and select gate-word line, cases.

The biasing voltages for select gate-select gate stress can be differentcompares to word line case, both to reflect the different levels thatare used on these different transistors and also on the differentdecoding options that may be available on the selects of the device.Although the drain and source select transistor sets are typicallyoperated respectively as a single drain and source transistor, if someor all of the elements of each set of (in this example) four can beindividually biased, if the needed select gate decoding is available,this can be used for applying a strip pattern, either AC or DC, betweenthe select gates lines of the same finger, different fingers of the sameblock, adjacent fingers of different blocks, or some combination ofthese. In the case where the set of select gates are share a gatecontact (or, similarly, for only a single such gate), these will allhave the same bias level and the stress is only applied between fingers(whether the same or different blocks). In an intermediate sort ofsituation where, say, one source select gate line (“SGSB”) canindependently biased while the other three (SGS) are connected togetherto have the same control, several stress options are available, such as:SGSB-SGSB (adjacent fingers); SGS-SGSB shorts (same finger, as for aword line to word line short); and SGS (either of 3 SGS of onefinger)-SGS (either of 3 SGS of adjacent finger).

Word Line to Local Source Line Shorts

In an array with a NAND type of architecture, the NAND string of a blockare typically connected to a common source line, as shown at 34 of FIG.5. The source lines of multiple blocks, even all of the blocks of a die,often share such a common source line. Referring to the 3D structure ofFIG. 11, in the arrangement shown there, between each of the fingers ofthe same block a local common source line CELSRC (or local interconnect,LI) runs up to connect the source side of the NAND strings to one ormore global common source lines (not shown in FIG. 11) running acrossthe top the structure. As can be seen in FIG. 11, this places the wordlines (and select gate lines) in proximity to this local source lineinterconnect, leading to the possibility of shorts across theintervening oxide.

During an erase operation in this sort of structure, the LI (CELSRC)will couple to the high erase voltage, while the word lines (and anydummy word lines) are low (0V) and the select gate lines can be eitherdriven or floated to prevent them from being erased. In case of an LI toword line short, the erase voltage will be droop and the device may notbe able to successfully erase the word lines. This defect can also causeread and program operations to fail. This is block level failure. Thissection looks at methods for determining such defects at test time.

In 2D NAND devices, there are often modes that apply a stress (highvoltage) on word lines, while keeping the source line low (close to 0V),but this sort of stress mode can degrade the characteristics of thememory cells, leading reliability and endurance concerns. Additional, ina typically 2D arrangement, the metal line of a source line does not runnext to word lines, so that the failure mode considered in this sectionis more specific structures, such as the BiCS array, that have this sortof lay out.

In the exemplary embodiment, a high voltage (˜VH) is applied on thelocal common source line of the block (LI or CELSRC), and lower level isapplied to the word lines, including any dummy word lines, with theselect gate lines either driven or floated. Here VH is a voltage levelsuch that it is high enough to break weak oxide between LI and any wordlines, but small enough such that reverse bias diode between the CPWELL(p+) and the CLESRC (n+) region does not break down; for example, in theexemplary embodiment this can be on the order an erase voltage, say 20V.The low level can be taken from among the low levels on the chip, suchas VWL, VSG, or VL. Both VWL and VSG are also voltage levels close toVSS or VSS, with values such that they will not stress the memory cells,so that endurance and reliability are not adversely affected. In thisexample, VSG is VSG voltage is mainly a biasing voltage to turn on theNAND string during read/program/erase operations and VWL can be mainlybe VCGRV (control gate read-verify) level, where VCGRV voltage can go aslow as close to 1 V in an exemplary embodiment. The CPWELL level can beset at range of values between the high and low levels, as long ascombinations with the other voltages does not break down the reversebias diode between the CPWELL (p+) and the CLESRC (n+) region.

This arrangement of bias levels will stress weak oxide depositions,whether due to contamination or other defect, in the region between theword lines and the local source line interconnect in order to bringabout a short. This can cause the defect to manifest itself at testtime, rather than once the device is in use. As high voltage levels arenot placed on the word lines, the cells will not be stressed, avoidingadverse effects for reliability and endurance. FIG. 25 again shows aside view of the exemplary embodiment with an overview of the appliedvoltages.

In FIG. 25 the word line to local source line stress is applied in thecircled region. The low voltage levels on the leftmost finger and thehigh voltage on the vertical source line interconnect. The arrowsillustrate particular examples of locations of stress between a wordline and the source interconnect line where leakage could occur.

FIGS. 26 and 27 are an exemplary flow of a test mode to screen localinterconnect (LI) to word line physical shorts, similar to FIGS. 15 and16 above for the intra-block word line to word line case. The test modecan either be done as part of an initial test process or as a systemlevel countermeasure where, after some number of cycles and beforeerasing any blocks, the test can be done to catch any bad blocks beforeprogramming in any data. FIG. 26 is the stress phase and starts at 2601with entry into the text mode. At 2603 the determination is made as towhether a single block or multiple blocks are to be tested and at 2605or 2607 the corresponding select signals are sent out. The stressvoltages are then applied at 2609. As with the word line to word linecases, the duration of the stress is a settable parameter, as can be thevarious levels.

FIG. 27 illustrates an exemplary embodiment for the detection phase,where many of the specifics are as discussed with respect to FIG. 16.This begins with a program operation at 2701, which is monitored at2703. If the program fails for any blocks, these are marked bad (2705).If the program operation passes, a read can then be performed at 2707.The read status is then monitored at 2709 and the result can be checkedby, for example, comparing the data as read back with the data asstored. Any blocks that fail are marked bad at 2711. If the readoperation passes, an erase can then be performed at 2713. This ismonitored at 2715, with any failing blocked marked at 2717. If the eraseis passed (2719), the test mode is then exited (2721).

FIG. 28 is a schematic representation of some of the elements on thememory chip, similar to FIGS. 17 and 21, and with the correspondingelements similarly numbered. To enable the inter-block word line to wordline stress mode, a corresponding stress mode select signal is sent fromthe on-chip control logic to the charge pump, which in turn can supplythe needed voltage levels for the word lines and select gate lines tothe drivers in block 2115. The usual levels for CELSRC are suppliedthrough switch 2825, with high voltage VH from the charge pump can besupplied through a high voltage switch 2827, that also can get itscontrol gate voltage from the charge pump.

Bit Line to Low Voltage Signal Shorts

In the memory array, global bit lines span the structure connecting thememory cells to the sense amplifies used in sensing operations. Thisshown above in FIG. 18, for instance, where each NAND string isconnected to a bit line, and each bit line is connected to a NAND stringin finger. The sense amps are then located on the periphery of thearray. In the exemplary embodiment, and for simplifying this discussion,each sense amp connects to a single bit line. In other embodiments,where less than all of the bit lines are sensed at once (say, everyother or every fourth bit line being sensed concurrently), multiple bitlines (such as 2 or 4) are associated with each sense amp. As thespacing of bit lines is typically smaller than the area needed by thesense amp circuits, the sense amp circuits are often staggered relativeto one another in the chip's layout. This means that one bit line may beadjacent to the sense amp associated with another bit line.

During erase operations in some memory circuit designs, such as theexemplary BiCS type embodiment illustrated with respect to FIGS. 9-12,during an erase operation bit lines will couple to the erase voltage(which can be in the ˜20-24V range), taking the bit lines to a highvoltage. The sense amplifiers generally operate at lower voltages, suchas VSS (0V), the high logic level (VDD, in the ˜2-3V ranges), and, insome embodiments, a somewhat higher sense amp level used in pre-chargingbit lines for sensing operations (VDDSA, in the ˜4-5V range). (Moredetail on sense amp structures and the levels involved is given in USpatent publication number 2014-0003157.) Due to the proximity of the bitlines at high voltages with sense amp circuitry at relatively lowvoltages, the bit lines can short to the adjacent lower voltagecircuitry. In case of such a bit line to low voltage signal short duringdevice operations, the erase voltage (VERA) may droop and circuit willnot be able to successfully erase selected blocks. Even if the bit lineis repaired, this fault can still cause erase failure. Further, as thebit lines are global, this will be global defect for the portion of thearray (typically the entire plane) spanned by the defective bit line.

FIG. 29 is a schematic representation of the situation in the BiCScontext. A side view of several fingers of the array are shown, a pairof adjacent bit lines BLn 2905 and BLn+1 be explicitly shown at top. Inan erase operation, the CPWELL is biased to the high erase voltage Vera.The unselected NAND strings will transmit the high voltage to these bitlines. Each of bit lines BLn 2905 and BLn+1 2915 are respectivelyconnected an associated sense amp SAn 2901 and SAn+1 2911 though aswitch 2903 and 2913, where sense amps are connect to the switches by an“internal” part of the bit line BLIn, BLIn+1. During erase, the switches2903, 2913 are shut off, protecting the sense amps' circuitry andkeeping the high voltage contained. Due to the layout, however, one bitline (such as BLn) may be adjacent to the BLI portion, or other senseamp elements, associated with another bit line (such as BLn+1),resulting in a short as represented at 2909. Consequently, for any eraseoperation the high voltage Vera on the CPWELL will drain off through bitline BLn 2905. If the charge pump supplying the erase voltage in notable to keep up with this leakage, Vera will not be maintained and theerase operation will fail. Although illustrated for a 3D arrangement,this problem can also show up in other architectures, including in 2Darrays. As in the preceding sections, to detect such defects, a test canmimic the stress involved to force incipient failures and then perform adetection phase to see whether the circuit is able to perform the neededoperations.

In the stress operation, one or more bit lines are set to the highvoltages, while one or more adjacent internal bit lines (BLI) and/orassociated sense amp circuits are set to a lower sense amp voltage.Although this can be done by applying the high level directly to the bitlines, most memory circuits typically do not include such connections.Consequently, the exemplary embodiment establishes the high bit linevoltage as these would come about through normal circuit operations,namely from the CPWELL. This can be done in a 1-plane erase failurestress mode by applying the high voltage to the P-well (e.g., bias˜20-24V) and having all blocks unselected, so the bit lines will couplethrough the NAND strings to the high voltage, while concurrently settingthe sense amp nodes to the (relatively) low sense amp voltages(VDDSA/VDDNSS). The stress can be applied in either a DC mode for someduration, or in an AC by applying some number of pulses of givendurations to the P-well.

Any resultant bit line short would drain off the high voltage from theP-well, reducing its ability to effect an erase as the charge pumpsupplying the high voltage may not be able to keep up. This is used inthe detection operation of the exemplary flow by reducing the drivecapability of the charge pump and determining whether the erase voltagecan be held. For example, the pump clock can be set at the slower end ofits range and the Vera voltage being measure internally to detect anydroop due to a bit line high voltage-sense amp node low voltagedefect/leak. A comparator circuit can use a reference voltage forcompassion with the Vera level. Alternately, the pump clock can be setat its slowest and an auto-detect built in self-test (BIST) mode can bebased on the charge pump's ON time being, due to the leakage, longerthan typical. (More detail on charge pump systems is given in Ser. No.14/101,180 filed on Dec. 9, 2013, and a large number of references citedtherein.)

This technique will stress the bit line to low voltage node,accelerating the failure of defects, but without overly stress thememory cells of the array. FIG. 30 is similar to FIG. 29, but markedwith some of the voltage levels involved in the stress phase. Asdiscussed above, VH is a voltage level high enough to break weak oxidebetween the bit lines and the low voltage nodes, but small enough suchthat there no degradation of the select gates or any other circuitelements. LV is the relatively low voltages used by the sense amp nodes(e.g., VDDSA/VDD/VSS).

In the exemplary embodiment, the blocks are all unselected, with theNAND strings conducting between the P-well and bit lines. This stressmode is chosen so as to minimize effects on cell characteristics toavoid negative effects on reliability and endurance. The stress mode canalso be used at system level by applying the bit line to low voltagenode stress to catch full plane erase issues prior to programming datainto the array. The stress can be applied at time 0 or after somenumbers of program-erase cycles.

FIG. 31 is an exemplary flow for these test processes, beginning byentering the test mode at 3101. The stress is applied at 3103, where theduration and voltage levels can be settable parameters. If an AC stressmode is used, this will applied for N loops 3105, where N can also be asettable parameter. In the exemplary detection flow, the pump clock isset to its slowest (3107), all of the blocks in the plane are unselectedand the Vera is supplied to the P-well from the pump (3109), and valueof Vera on the P-well is measured (3111). At 3113 the P-well levelcompared against a reference level and if it has drooped, a flag can beset according (here, as high at 3119) and this status can be reportedout at 3121. If Vera is maintained, the flag is set (in this example) aslow at 3115 and the status reported out at 3117.

FIG. 32 is a schematic representation of some of the elements on thememory chip, similar to (and similarly number to) FIGS. 17, 21 and 28,for implemented the test mode of FIG. 31. During the detection phase asenabled by the stress mode select signal, while the detuned charge pumpdrives the P-well the switch SW 3221 supplies the P-well level to thecomparator 3223. This can be stepped down by the step down circuit 3225for comparison to a reference level Vref. The output of the comparator3223 can then be converted to a digital value at A/D 3227 and stored inthe register 3229 as the Flag value, where the register 3229 can receivea clock signal from the FSM & Sequencer block 3213 to latch the flagvalue. The flag value will indicate if the plane failed, and put out atthe I/O page 3231 (separated out here form the other pads forillustrative purposes).

CONCLUSION

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

It is claimed:
 1. A method of determining whether one or more blocks ofa semi-conductor memory device are defective, the memory devicecomprising a plurality of blocks, each formed of a plurality of NANDstrings having multiple memory cells connected in series between one ormore source select gates and one or more drain select gates, the memorycells being connected along word lines and the source and drain selectgates being respectively connected along source and drain select lines,where each of a block's NAND strings are connected though thecorresponding source select gates to a local source line, the methodcomprising: performing a stress operation on a selected block,comprising: applying a high voltage to the local source line; andconcurrently setting the word lines, source select lines and drainselect lines to a low voltage; subsequently performing a defectdetermination operation, including: performing a write operation on theselected blocks; and determining whether the write operation wassuccessful.
 2. The method of claim 1, wherein determining whether thewrite operation was successful comprises determining that the number ofcells along a word line that fail to verify exceeds a predeterminedlimit.
 3. The method of claim 1, wherein determining whether the writeoperation was successful comprises determining that the write operationfailed based on the number of programming loops exceeding a limit. 4.The method of claim 1, further comprising: in response to determiningthat the write operation on the selected block was not successful,marking the selected block as detective.
 5. The method of claim 1,wherein the detect operation further includes: subsequently performing aread operation on the selected block; and determining whether the dataread from the selected block is corrupted.
 6. The method of claim 5,wherein determining whether the data read from the selected block iscorrupted includes comparing the data as read with the data asprogrammed.
 7. The method of claim 6, wherein the data as programmed isa predetermined pattern of known random data.
 8. The method of claim 6,wherein determining whether the data read from the selected block iscorrupted includes determining that the number of incorrect bits in thedata as read exceeds a predetermined limit.
 9. The method of claim 5,wherein determining whether the data read from the selected blocks iscorrupted is based on an ECC result.
 10. The method of claim 5, furthercomprising: in response to determining that the data read from theselected block is corrupted, marking the block as detective.
 11. Themethod of claim 1, wherein the defect operation further includes:subsequently performing an erase on the selected block; and determiningwhether the selected block is successfully erased.
 12. The method ofclaim 11, further comprising: in response to determining that theselected block is not successfully erased, marking the block asdetective.
 13. The method of claim 1, wherein the memory circuit is amonolithic three-dimensional semiconductor memory device where thememory cells are arranged in multiple physical levels above a siliconsubstrate and comprise a charge storage medium.
 14. The method of claim13, wherein the NAND strings and the local source lines run in avertical direction relative to the substrate, and the word lines andsource and drain select lines run in a horizontal direction relative tothe substrate.
 15. The method of claim 14, wherein the local sourcelines of multiple blocks are connected to a global source line runningin a horizontal direction relative to the substrate.
 16. The method ofclaim 1, wherein the stress operation and subsequent defectdetermination operations are performed as a built in self-test process.17. The method of claim 1, wherein the stress operation and subsequentdefect determination operations are performed after the memory circuithas been operated over a number of erase cycles.
 18. The method of claim1, wherein the high voltage level is within the range of programmingvoltage used during write operation.
 19. The method of claim 1, whereinthe low voltage level is ground.
 20. The method of claim 1, wherein thelow voltage level is sufficient to turn on the select transistors of theNAND strings.
 21. The method of claim 1, wherein the low voltage levelis sufficient to turn on the memory cells of the NAND strings.